(CNN) — When Chelsie Shellhas saw a tall young man slip into a storage locker and shut the door behind him one evening this week, it just looked wrong. She dialed 911.

Her instincts were right: Behind the door were bomb-making materials, police in Wacesa, Minnesota, said Thursday. Her call thwarted a cunningly planned gun and bomb attack at the boy’s school, they say.

John David LaDue, 17, was looking to carry it out in the coming weeks, police said in an official statement.

After Shellhas’ call on Tuesday, officers arrived at the storage park to find LaDue among an assortment of gunpowder, pyrotechnic chemicals, ball bearings and a pressure cooker, police said.

CNN does not usually publish the names of minors charged with crimes but is naming LaDue in this case, because his name has been widely reported in his community.

Police locked up the storage bay and took LaDue in for a voluntary interview. He shared a macabre plan, police said.

His tale and recovered evidence resulted in charges of four counts of attempted murder, six counts of explosives possessions and related property damage charges.

He is being charged as a juvenile, said prosecutor Brenda Miller.

Alleged plan: Kill the family first

First, LaDue was planning to kill his father, mother and sister, police said.

He was particularly close to his sister, their music tutor said. Ryan Lano gave LaDue guitar lessons for four years. “He would almost always come in with his sister, who played the drums,” he said.

After their slayings, the teen, who idealized the Columbine school shooters, allegedly wanted to set a fire.

The arson, however, would only be a diversion to tie up emergency responders while he carried out the main plot at his school, police said.

He’d planned to set off “numerous bombs” during the lunch hour at Waseca Junior/Senior High School, then kill the school resource officer, police said.

Then he’d unleash gunfire on students until police officers arrived, allegedly planning to let them kill him.

Armed to the teeth

The teen’s cache in the storage locker contained just pieces of the puzzle.

Police got a search warrant and at 10:30 p.m. arrived at LaDue’s home, where he lived with his family.